In a step of producing printing plates, recently, photographic light-sensitive materials which can be handled in bright room in spite of using silver halide as the photographic element are being developed which meet the requirement of performing a contact exposure (so-called contact work) using a relatively low-speed light-sensitive material. Such results are attained by exposing a light-sensitive material having greatly reduced light sensitivity (about 1/10.sup.4 to 1/10.sup.5 that of ordinary sensitivity) to visible light to a light source containing a high proportion of ultraviolet (e.g., a very high pressure mercury lamp, a metal halide lamp, etc.), under a safe light containing substantially no ultraviolet.
On the other hand, in contact work, simple one sheet contact work (i.e., a nega/posi image conversion is performed contact-exposing one image-exposed and developed photographic film as an original and a light-sensitive material for contact work) and high-level image-conversion work for making so-called white lettering, on a solid background or a white-on-black headline are performed. The term "white lettering on a solid background" means uninked portions of letters, marks, etc., in a dot-like inked pattern on paper (dotted portion) or a wholly inked portion on paper (called "a solid black portion") in printed material.
The manner of making white lettering on a black background in producing printing plates will now be explained more practically.
As shown in FIG. 1, a base 1 has adhered thereto a developed film 2 (line image) having line positive images such as letters, marks, etc., is disposed on a transparent or translucent base 3 (usually a polyethylene terephthalate film a few hundred .mu.m in thickness) which has adhered thereto a developed film (dot image original) which has dot images. The dot image portion of the assembly is brought into intimate contact with an emulsion surface of a light-sensitive material 5 for contact work followed by light-exposure development to form white line image portions in the dot images.
An important point in the above procedure is that the dot image and line image must be subjected to a nega-posi image conversion according to the dot area and the line width, respectively. For example, a dot image having 50% black area must be correctly converted into a dot image having a 50% white area and a line image having a black line width of 50 .mu.m must be correctly converted into a line image having a white line width of 50 .mu.m. However, as is clear from FIG. 1, the dot image is exposed to the light-sensitive material for contact work in a state of intimate contact with the emulsion surface of the light-sensitive material, while the line image is exposed to the light-sensitive material through the dot image original 4 (usually having a thickness of about 110 .mu.m) and the base 3 therefor (usually having a thickness of 100 .mu.m). In other words, the line image is exposed to the light-sensitive material for contact work as a vague line image since exposure is carried out through transparent or translucent spacers a few hundred .mu.m thick. Accordingly, when conventional exposure amount (the exposure amount to faithfully nega/posi convert a dot image) is applied, the white line width of the line image becomes narrow by the influence of the diffused exposure. On the other hand, when the exposure amount is reduced to reducing the influence of the exposure to faithfully perform the nega-posi conversion of the line width of the line image, the dot area is reduced due to the insufficiency of the exposure.
Further, an attempt to reducing the influence of the exposure through multiple substrates to improve the quality of white lettering on solid background encounters the problem that adhering traces of a tape used to fix the line image or dot imates on the base and pin hole marks are liable to appear.
Also, a silver halide light-sensitive material for contact work in a bright room is liable to form pin holes due to dust, etc., when it is exposed to ultraviolet rays as compared with conventional light-sensitive materials for contact work in a dark room.
Means for preventing a reduction of the quality of white lettering on a black background and a reduction in image quality due to the formation of adhering tape traces and pin hole marks have scarcely been reported until now since the above-described procedures are different from conventional improvements in photographic characteristics (e.g., sensitization, improvement of dot image quality, increasing the contrast of a characteristic curve, etc.), directed to improving a light-sensitive materials for making printing plates.